1. Field of the Invention
The present invention relates to a method for adapting power amplifier predistortion by using compression detection feedback.
2. Description of the Related Art
In wireless systems a power amplifier is frequently used as one of the final transmitter stages for the purpose of amplifying the transmitted waveform to the appropriate level to provide adequate signal-to-noise ratio over the link. To achieve reasonable efficiency from a power amplifier, the amplifier is often operated near its compression point. Yet at an operating output power at which the amplifier operates efficiently, the amplifier often undesirably generates a level of distortion on the waveform that corrupts the modulation accuracy of the waveform and generates potentially excessive noise in adjacent carrier regions.
Contemporarily, many forms of linearization have been developed in an effort to recover the modulation accuracy of the power amplifier while operating the power amplifier near the compression point in order to achieve the required efficiency. One such form of linearization is digital predistortion.
In various developing wireless standards such as 802.16e and Third Generation Partnership Project 2 (3GPP2) Long Term Evolution (LTE), Orthogonal Frequency-Division Multiplexing (OFDM) waveforms are used with high order quadrature modulation and multiple subcarriers. This form of modulation requires very good modulation accuracy in order to achieve an adequate bit error rate (BER) or frame error rate (FER). Even in the uplink path, an error vector magnitude (EVM) of less than 3% is required from the terminal transmitter. An ideal amplifier would be a totally linear device, but real amplifiers are only linear within certain practical limits. When an input signal is increased, the output also increases until a saturation point is reached where some part of the amplifier becomes saturated and cannot produce any more output; this is called clipping, and results in distortion. Without any form of linearization, a signal can only be transmitted within the linear operating region of the power amplifier with very poor efficiency. Another effect is that the OFDM waveforms used in these standards often have high peak-to-average ratios (PAPR) exceeding 9 dB. Therefore, in order to avoid saturating the power amplifier at the peaks of the waveform envelope, the average output power must be maintained at close to 9 dB below the saturation point of the amplifier.
Between the high PAPR of the OFDM waveform and the low EVM requirements of the uplink, the net result is that the terminal power amplifier operates at a relatively poor efficiency. Whereas terminal power amplifiers operated in Global System for Mobile communications (GSM) or Code division multiple access (CDMA) systems frequently achieve power added efficiencies (PAE) of 40 to 50% at a maximum rated power, the terminal power amplifiers used in mobile WiMax (802.16e) and LTE achieve a PAE of around 20% or less. Therefore to transmit a carrier at a comparable power level, more than twice the amount of DC power is required by the power amplifier. This causes a much greater drain on the terminal battery thereby causing a relatively short battery life. The short battery life effect has been widely experienced in broadband networks currently in operation.
One means to improve the terminal power amplifier efficiency and extend the battery life is adding digital predistortion to the terminal. This form of linearization allows the power amplifier to be operated at a lower output back-off since the linearization can then be used to correct the degraded EVM and Adjacent channel Power Ratio (ACPR) performance back to an acceptable level. Historically, digital predistortion has not been widely implemented in terminal transmitters because the power dissipation of the added signal processing circuitry has more than offset the reduction on power amplifier current drain. Therefore digital predistortion within wireless systems has often been limited to the base station transmitter operating at 10 to 40 watts. In this application the 2 or 3 watts of power dissipated by the predistortion circuitry is more than offset PA efficiency improvement realized. Yet it can be seen that if the power dissipation of the predistortion circuitry can be reduced to 100 mW or less, that a net improvement would be possible even in a terminal transmitter operating at ½ watt if the PAE of the power amplifier could be simply increased from 20% to 25%.